This is a U.S. national stage application of International application No. PCT/AU98/00104, filed Feb. 19, 1998, the benefit of the filing date of which is hereby claimed under 35 U.S.C. xc2xa7 120, which in turn claims the benefit of Australian application No. PO 5239, filed Feb. 20, 1997, the benefit of the filing date of which is hereby claimed under 35 U.S.C. xc2xa7 119.
The present invention relates to a method of marking articles by a laser and more particularly to a method of marking security documents or other documents having a clear substrate covered by opacifying layers, such as printed matter.
Previously known are methods for marking glass or plastic materials using a laser. A high energy laser beam is made to converge at a plane underneath the surface of the material to form an opaque region through ionisation of the material at the point of convergence. In another known arrangement, marking of plastic objects by a coloured laser is performed by exposing a lacquered film, which is applied to the surface of the object, to laser radiation. The film, containing at least one colour component, softens at the irradiated areas and the colouring component penetrates the surface and the non-irradiated areas of the film are dissolved by a suitable solvent to leave a darker impression where the markings are located. Also known are security papers, such as banknotes, cheques and identity cards that have luminescent authenticity marks made with substances that luminesce only in the visible spectrum.
The surfaces of laser-sensitive plastics, such as polyvinyl chloride (PVC), have been marked by causing a colour change in the irradiated area of the surface. Where a particular plastic is not laser-sensitive, such as polypropylene (PP), polyethylene (PE) or polyethyltetraphthalate (PET), it can be made laser-sensitive by incorporating a laser-sensitive additive into the plastic.
Yet another arrangement discloses a lithographic printing plate, imageable by laser, which has a first layer and a substrate layer underneath the first layer. The substrate has efficient absorption characteristics of infra red laser radiation, the first layer and substrate layer each having different affinities for ink (in a dry-plate construction) or an adhesive fluid for ink (in a wet-plate construction). When irradiated by the laser, the substrate absorbs the radiation and ablation occurs at the surface of the substrate which is in contact with the first layer. This leads to loosening of the substrate overlying the first layer which is then removed at the point of exposure. The result of the removal is an image spot whose affinity for the ink or ink-abhesive fluid differs from that of the unexposed first layer leading to a permanent marking.
Whilst the methods of marking articles described above are useful for marking one side or surface of an article, if it is desired to mark opposite sides or surfaces of an article, two separate laser marking operations are required. It is therefore desirable to provide a simple and effective method of marking opposite surfaces of an article, such as a security document.
It is also desirable to provide a convenient method of forming a transparent window in an article such as a security document.
According to one aspect of the present invention there is provided a method of marking an article, the article comprising a substrate with opacifying layers on opposite surfaces of the substrate, said method comprising:
irradiating an area of the opacifying layer on one surface of the substrate with laser radiation such that said area of the opacifying layer on said one surface is ablated by the laser radiation to produce a marking by removing said area of the opacifying layer on said one surface, wherein the laser radiation travels through the substrate and ablates an area of the opacifying layer on the opposite surface of the substrate to produce a marking by removing the area of the opacifying layer from the opposite surface of the substrate.
The present invention also provides a method of creating a transparent window in an article comprising a clear substrate with opacifying layers on opposite surface of the substrate, said method comprising:
irradiating an area of the opacifying layer on one surface of the substrate with laser radiation such that said area is ablated by the laser radiation to remove said area of the opacifying layer from said one surface, wherein the laser radiation travels through the substrate and ablates an area of the opacifying layer on the opposite surface of the substrate to create a transparent window in the substrate.
Preferably, the opacifying layers on opposite surface of the substrate have greater absorption characteristics for laser radiation than the substrate. Preferably the wavelength of the laser radiation is selected such that when the article is exposed to radiation the opacifying layers absorb the radiation and are ablated from the substrate, the substrate being substantially transparent to the radiation at that selected wavelength so that the laser radiation travels through the substrate substantially unimpeded.
The laser radiation may be a continuous beam. Alternatively, it may be emitted as a pulse or series of pulses.
The present invention also provides an article comprising a substrate having opacifying layers on opposite surfaces of the substrate, the substrate being formed from a material which is substantially transparent to laser radiation of a selected wavelength, wherein each of said surfaces has a marking formed therein, said markings being formed by removing a first area of the opacifying layer on one surface of the substrate by ablating said first area with laser radiation of the selected wavelength, and allowing the laser radiation to pass through the substrate to remove an area of the opacifying layer on the other surface in register with said first area removed from the opacifying layer on said one surface.
This invention generally relates to printed articles, such as banknotes or currency, security documents or any other document having printed matter arranged thereon, that have a clear substrate with opacifying layers of printed ink on opposite surfaces of the substrate on at least a portion of the article. Laser radiation may be used to create a clear or transparent area, in the shape of a particular design or character or symbol, in the portion of the article where there are printed ink layers on one or both sides of the clear base substrate. It does this by removing or ablating the printed ink layers from either or both sides of the substrate in the appropriate areas of the article.
A certain relationship must exist between the wavelength of the laser light used and the relative absorption characteristics of the clear substrate and the layers of printed matter at the selected wavelength. For example, the layers of printed matter should be good absorbers of the laser radiation so that the layers can be removed or ablated when exposed to the radiation for a predetermined period. Also, the clear substrate should be a poor absorber of laser radiation when compared to the absorption characteristics of the printed layers at the selected wavelength. The substrate should be substantially transparent to the radiation at this wavelength, allowing light at the selected wavelength to travel through the substrate without creating substantial damage or distortion to the clear substrate. In this way, a transparent window is formed with both surfaces of the substrate being in register according to the desired shape, symbol, design or character at the places of exposure to the radiation.
The present invention is particularly, but not exclusively, applicable to banknotes or other security documents having at least a portion of the document formed from a clear plastics substrate having at least one opacifying layer of ink on both of its surfaces. The clear plastics substrate is preferably formed of a transparent polymeric material, such as PE, PP or PET, which may be made up of at least one biaxially oriented polymeric film. The substrate may comprise a single layer film of polymeric material. Alternatively, the substrate may comprise a laminate of two or more layers of transparent bi-axially-oriented polymeric film of the type described in Australian Patent No. AU-A-87665/82, the contents of which are incorporated herein by reference.
The opacifying layers of printed matter may comprise any one or more of a variety of opacifying inks which can be used in the printing of banknotes or other security documents. For example, the layers of opacifying ink may comprise pigmented coatings comprising a pigment, such as titanium dioxide, dispersed within a binder or carrier of cross-linkable polymeric material as described in Australian Patent Specification No. AU-A-87665/82.
Preferably, the selected wavelength of the laser radiation falls substantially within the range from about 0.5 microns to about 20 microns.
In a particularly preferred embodiment, the clear plastics substrate is formed of PP, the layers of opacifying ink are formed of TiO2 and silica dispersed within a polyurethane based resin, and the wavelength of the laser radiation used is either approximately 1.06 microns or approximately 10.6 microns.